GaN is one of the most interesting semiconductor materials that is useful for fabrication of laser diodes [LD], light emitting diodes [LED] and high power, high frequency electronic devices. But, due to the severe shortage of GaN substrates, these devices are fabricated on foreign substrates such as SiC, Sapphire, LiGaO2 or LiGaO3. Hence, they are suffering from high dislocation density and bending caused by lattice mismatch and thermal expansion coefficient mismatch respectively between GaN and substrate. In order to improve the performance of devices such as LDs, LEDs and high electron mobility transistors (HEMTs), homoepitaxial device layers grown on high quality free standing GaN substrates are very important.
GaN nitride materials are finding many applications such as microwave electronics, electro-optics and power electronics. In addition many of the unique properties of GaN such as wide bandgap (leading to violet and ultraviolet LED's and lasers) as well as large spontaneous polarization are being exploited for new and novel devices. In order to continue and speed the development of this technology bulk substrates are desired. Bulk substrates will allow the realization of high device efficiencies lower leakage currents and longer device lifetimes.
Several efforts are underway to produce freestanding GaN substrates. Growth rate of GaN layers obtained by conventional methods such as MOCVD or MBE is very low and is not suitable for producing bulk GaN single crystals. In the recent past, Hydride Vapor Phase Epitaxy, High Nitrogen Pressure Solution Growth, and Sublimation Techniques are being used to grow bulk GaN crystals. Free standing GaN crystals are also prepared by reaction of Ga vapors with ammonia, Ga with Ammonia, sublimation of GaN powder and growth in sodium flux.
Initial development of GaN devices has utilized foreign substrates and hetero-epitaxy. These substrates include sapphire, ZnO, LiGaO2, LiAlO and silicon carbide. Devices fabricated from hetero-epitaxial materials suffer from problems relating to dislocations, domains and grain boundaries. In additions problems of cracking relating to thermal mismatches in the materials are evident. In order to ameliorate the problems several patents have been filed relating to techniques of substrate removal and hetro-epitaxal growth which are designed to reduce these problems.
In order to completely eliminate these problems a single crystal boule of GaN is required. One technique, GaN is grown from a liquid held at high pressure. This technique produces high quality platelets of GaN but results in a very low growth rate and utilizes expensive high pressure growth technology which is not easily scaled up.
There have been several techniques which utilize Hydride Vapor Phase Epitaxy (HVPE) or Metallo Organic Chemical Vapor Deposition (MOCVD) technology to produce bulk films. These techniques employ Ga transport via GaCl or a Ga containing metal organic compound. The MOCVD based techniques have not demonstrated high growth rates. The HVPE based techniques show relatively high growth rate but are limited in substrate temperature due to the details of Cl based transport. In addition these HVPE techniques suffer from significant loss of growth species due to wall depositions
There are also methods in which GaN is grown from Ga vapor. These methods utilize Ga obtained from sources held at high temperatures in the presence of ammonia. In these methods it is not possible to stabilize or control the Ga vapor. The substrate temperature is maintained lower than the vapor source.
Growth of bulk GaN has been demonstrated using liquid phase based techniques. High growth temperatures and pressure (10-20 kilobars) are employed in an attempt to overcome an extremely low solubility of nitrogen in melts in general and Ga melts in particular. Despite the high pressures, nitrogen solubility is still low and growth rates no greater than 0.01-0.05 mm/hr can be obtained. In addition to the low growth rates this production process is very difficult to implement due to the high pressures. Similar problems of solubility are seen in flux based systems in which additives are used to increase the nitrogen solubility in the melt. Amonothermal growth which uses super critical ammonium (similar to the process for the production of single crystal quartz) suffers from even lower growth rates and also has not proven to be commercially feasible.
All of the competitive bulk growth technologies for WBG semiconductors are vapor based. In the case of GaN there have been several techniques which utilize HVPE or MOCVD technology to produce bulk films. These techniques employ Ga transport via GaCl or a Ga containing metal organic compound. The MOCVD based techniques have not demonstrated high growth rates. More promising are the HVPE based techniques which show relatively high growth rates (the best reported are 200 um/hr) but are limited in substrate temperature due to the details of Cl based transport. Prior art FIG. 1 shows values of the growth rates as well as the results of simulations of GaN growth in a prior chloride transport system as a function of substrate temperature. The growth rate is almost constant in the temperature range (T=1050-1100° C.) and dramatically drops down at higher temperatures due to etching of GaN by HCl and H2. In addition these HVPE techniques suffer from significant loss of growth species due to wall depositions.
There have been several studies related to the growth of GaN from Ga vapor. In some prior attempts, growth rates of 1 mm/hr have been demonstrated, but it was not possible to stabilize the growth over long periods of time.